Because nearly every ebook reader on the iPhone has its own store (and the ones that don't have the problem of not having their own store), what's best is very subjective. That said, we feel Kindle is your best option at the moment because it doesn't tie you to a specific platform, has a wide selection of books, and will let you read some other file formats.

Features

Easily buy a book or send a sample directly to your iPhone from Amazon's online store

Only store the books you want

Convert documents over email for viewing with Kindle

Make notes in books or documents

Bookmark locations for viewing later

Simple, straightforward interface

Very large selection of ebook content through Amazon.com

Only keep the books on your device that you want and archive the rest, available for download at any later date

Adjust font size

Easily jump to different locations in a book or document

Where It Excels

Kindle was the first major ebook platform so it has had a good amount of time to mature and do things well. It's certainly not perfect, but buying an ebook and managing your content is ridiculously simple. Kindle can also support a few other document types (such as Microsoft Word docs, text files, and PDFs) through email conversion. While that may seem tedious, it's often a lot faster than plugging in your device and transferring the files manually. Whatever you choose to read, Kindle provides a pretty good experience. You can select text size, change the background and text colors for better reading under certain lighting conditions, jump around to different chapters or sections of a book, and search for content. It's easy to use, has a nice and focused feature set, and plenty of content that's easy to access.

Where It Falls Short

As mentioned at the very beginning of this post, no ebook reader is really all that great. Like most other options, Kindle's store is filled with DRM-protected content and that comes with a variety of restrictions. For example, only very recently did Amazon offer the ability to copy selections of text from some books, and that's still not possible on the iPhone version (at the time of this writing). When you buy content from Amazon, you're stuck with Amazon. We think this is easier to accept because Amazon has developed ways for you to read Kindle content on virtually any computer or mobile device. You can even read in your web browser if you've managed to find a platform Kindle doesn't directly support. But you're still locked down to Amazon nonetheless. In my experience with Kindle, it also tends to have a few hiccups here and there when transferring media. These issues have always been easy enough to resolve, but they do happen with more frequency than I experienced with other ebook apps on iOS and other platforms.

The Competition

Kobo (Free) is a very nicely designed ebook app for reading free books. It boasts the availability of one million free and affordable books. It also has support for Instapaper and makes it really easy to both share content and get content on your device.

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iBooks (Free) is the ereader you'd expect from Apple. It sports a great interface, is simple to use, and lets you easily by books directly from the app. As far as the user interface is concerned, most will consider it better than Kindle. (I don't like it, personally, but I seem to be in the minority here.) It can also read PDFs without the need to email them (although that's not to say it's any less tedious to get them into the app). With all of that said, the iBooks store tends to have higher prices than you'll find at Amazon and if you buy from iBooks, your content is forever stuck on the Apple mobile platform. You can't even read them on your Mac, which may not be the ideal experience for some but is helpful with reference books and for the small minority who does like reading on a computer screen. The content lockdown is really the primary reason why iBooks isn't the top choice. We don't like the DRM any of the stores provide, but at least Amazon offers a means to port the content to practically any device.

eBookMobi ($2) is a hybrid app (meaning it works on all iDevices natively) that supports multiple languages and reads comic books as well as ebooks. The multi-language support will let you read foreign language content and translate it into your primary language if you're struggling to understand a word. It supports non-DRM Palm OS ebooks, non-DRM ePub ebooks, PDF files, RTF files, Word documents, CHM files, and a variety of comic/manga formats. If you need a very versatile app and don't care about a bookstore, eBookMobi is $2 well spent.

Nook (Free) is what you get if you've already purchased ebook content from Barnes and Noble's ebook store and want to be able to view it on your iPhone. There aren't many other reasons why you'd use this app.

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Got another ebook reader you love that wasn't mentioned here? Share it in the comments!

Lifehacker's App Directory is a new and growing directory of recommendations for the best applications and tools in a number of given categories.